Selective Aerobic Oxidation of Alcohols with a Combination of a Polyoxometalate and Nitroxyl Radical as Catalysts

Ben‐Daniel, Revital; Alsters, Paul L.; Neumann, Ronny

doi:10.1021/jo0105843

Cited by 143 publications

(65 citation statements)

References 18 publications

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“…The observed facile conversion of TEMPOH to TEM-PO in air contrasts with the results of Neumann and coworkers [90] who studied the oxidation of alcohols catalyzed by TEMPO in combination with a heteropoly acid, H 5 PV 2 Mo 10 O 40 . They postulated that the oxidation of TEMPOH to TEMPO is the slower, rate-determining step.…”

Section: ð14þ ð15þcontrasting

confidence: 88%

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Organocatalytic Oxidations Mediated by Nitroxyl Radicals

2004

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The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as the archetypal TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). The different types of reactivity observed are rationalized by considering the bond dissociation energies (BDEs) of the respective N-hydroxy precursors, substrates and reaction intermediates. Reactive diacylnitroxyl radicals are generated in situ from the corresponding N-hydroxy compound. The protagonist, NHPI, catalyzes a wide variety of free radical autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal co-catalysts improved conversions and selectivities are obtained in the autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. For example, cyclohexylbenzene afforded the 1-hydroperoxide in 97.6% selectivity at 32% conversion when the autoxidation was performed in the presence of 0.5 mol % NHPI, and the product hydroperoxide as initiator, at 100 8C. This forms the basis for a potential coproduct-free route from benzene to phenol. In combination with transition metal co-catalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin NHS, afford effective catalytic systems for the effective autoxidation of hydrocarbons, e.g., toluenes to carboxylic acids, under mild conditions. In the case of the less reactive cycloalkanes, NHS proved to be a more active catalyst than NHPI which is attributed to the higher reactivity of the intermediate nitroxyl radical, resulting from the replacement of a carbonyl group in NHPI by the more strongly electron-attracting sulfonyl group. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g., alcohols, with single oxygen donors such as hypochlorite and organic peracids. The reactions involve the intermediate formation of the corresponding oxoammonium cation as the active oxidant. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols. These reactions involve metal-centered dehydrogenations and the role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu) and oxidation of the alcohol (Cu) via hydrogen abstraction or one-electron oxidation processes. Detailed mechanistic investigations, including kinetic isotope effects, revealed that oxoammonium cations are not involved as intermediates in these reactions. In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme. Similarly, N-hydroxy compounds such as ...

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Section: ð14þ ð15þcontrasting

confidence: 88%

“…In this case regeneration of TEMPO, by metal mediated oxidation of TEMPOH, may be rate-limiting as proposed by Neumann and coworkers. [90] The total reaction scheme is shown in Figure 22.…”

Section: ð14þ ð15þmentioning

confidence: 99%

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

2004

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“…[67] There are many reports on the aerobic oxidation [68] of alcohols by using both TEMPO and transition metals as cocatalysts. [69] Ceric ammonium nitrate (CAN), [70] H 5 PV 2 Mo 10 O 40 ·34 H 2 O, [71] FeCl 3 /NaNO 2 , [72] [RuCl 2 (PPh 3 ) 3 ], [73] and the couple Mn(NO 3 ) 2 /Co(NO 3 ) 2 [74] have been used successfully as co-catalysts in aerobic TEMPO-mediated oxidations. Without doubt, copper-based co-catalysts have been most intensively studied in this regard.…”

Section: Oxidation Of Alcoholsmentioning

confidence: 99%

Nitroxides: Applications in Synthesis and in Polymer Chemistry

2011

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This Review describes the application of nitroxides to synthesis and polymer chemistry. The synthesis and physical properties of nitroxides are discussed first. The largest section focuses on their application as stoichiometric and catalytic oxidants in organic synthesis. The oxidation of alcohols and carbanions, as well as oxidative C-C bond-forming reactions are presented along with other typical oxidative transformations. A section is also dedicated to the extensive use of nitroxides as trapping reagents for C-centered radicals in radical chemistry. Alkoxyamines derived from nitroxides are shown to be highly useful precursors of C-centered radicals in synthesis and also in polymer chemistry. The last section discusses the basics of nitroxide-mediated radical polymerization (NMP) and also highlights new developments in the synthesis of complex polymer architectures.

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“…[3a] Therefore, the pursuit of using molecular oxygen or air as the terminal oxidant has attracted intense research interest, and many highly efficient catalyst systems have been developed for selective aerobic alcohol oxidation using transition metal catalysts [4] (mainly copper, [3] palladium, [5,6] and ruthenium [7] ), alone or in combination with stable nitroxyl free radicals (e.g., 2,2,6,6-tetramethyl-piperidyl-1-oxy, TEMPO). [8,9] The introduction of TEMPO as a co-catalyst in the transition-metal-based catalytic systems was effective in improving the reaction selectivity under mild conditions. However, deactivation of the transition metal catalysts in these aerobic oxidation systems has been a recurring problem due to water competing with the substrate and/or intermediate for vacant coordination sites on the active metal catalysts.…”

Section: Xinliang Wangmentioning

confidence: 99%

TEMPO/HCl/NaNO₂ Catalyst: A Transition‐Metal‐Free Approach to Efficient Aerobic Oxidation of Alcohols to Aldehydes and Ketones Under Mild Conditions

Wang

Liu

et al. 2008

Chemistry A European J

192

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Hydrochloric acid, a very inexpensive and readily available inorganic acid, has been found to cooperate exquisitely with NaNO(2)/TEMPO in catalyzing the molecular-oxygen-driven oxidation of a broad range of alcohol substrates to the corresponding aldehydes and ketones. This transition-metal-free catalytic oxidative conversion is novel and represents an interesting alternative route to the corresponding carbonyl compounds to the metal-catalyzed aerobic oxidation of alcohols. The reaction is highly selective with respect to the desired product when carried out at room temperature in air at atmospheric pressure. Notably, the use of very inexpensive NaNO(2) and HCl in combination with TEMPO for this highly selective aerobic oxidation of alcohols in air at ambient temperature makes the reaction operationally and economically very attractive. The results of mechanistic studies, performed with the aid of electrospray ionization mass spectrometry (ESI-MS), are presented and discussed. TEMPO, TEMPOH, and TEMPO(+) were observed in the redox cycle by means of ESI-MS. On the basis of these observations, a mechanism is proposed that may provide an insight into the newly developed aerobic alcohol oxidation.

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Selective Aerobic Oxidation of Alcohols with a Combination of a Polyoxometalate and Nitroxyl Radical as Catalysts

Cited by 143 publications

References 18 publications

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Nitroxides: Applications in Synthesis and in Polymer Chemistry

TEMPO/HCl/NaNO₂ Catalyst: A Transition‐Metal‐Free Approach to Efficient Aerobic Oxidation of Alcohols to Aldehydes and Ketones Under Mild Conditions

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Selective Aerobic Oxidation of Alcohols with a Combination of a Polyoxometalate and Nitroxyl Radical as Catalysts

Cited by 143 publications

References 18 publications

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Nitroxides: Applications in Synthesis and in Polymer Chemistry

TEMPO/HCl/NaNO2 Catalyst: A Transition‐Metal‐Free Approach to Efficient Aerobic Oxidation of Alcohols to Aldehydes and Ketones Under Mild Conditions

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Product

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TEMPO/HCl/NaNO₂ Catalyst: A Transition‐Metal‐Free Approach to Efficient Aerobic Oxidation of Alcohols to Aldehydes and Ketones Under Mild Conditions